Deciphering the Molecular and Physiological Mechanisms Underlying Abiotic Stress in Cultivated Crops

Global agricultural systems face the challenge of achieving sustainable intensification to ensure food security for a nearly ten billion global population by 2050 while operating within constrained planetary boundaries. This endeavour is hampered by climate change, soil degradation, and inefficient resource use, which exacerbate abiotic stresses and diminish crop yields. Mitigating abiotic stress requires a multi-pronged strategy such as characterising innate genetic tolerance, deepening mechanistic understanding of stress adaptation, and integrating innovative tools, such as plant biostimulants, to enhance crop performance and reduce ecological costs. However, adoption of these products is often hampered by variable efficacy and a limited mechanistic understanding of their modes of action. This thesis addresses a critical knowledge gap by developing a tripartite strategy for abiotic stress mitigation: (i) multi-omics characterization of alkaline stress tolerance in tomato; (ii) systems-level dissection of adaptations to phosphate starvation in sugar beet; and (iii) rigorous evaluation of lignosulfonate-based biostimulants to enhance nitrogen-use efficiency in maize. These objectives are pursued through an integrative approach combining physiology, biochemistry, and molecular biology to advance a predictive understanding of abiotic stress across three globally significant crops. The first study employs an integrated multi-omics approach to dissect the molecular and physiological basis of alkaline stress tolerance in tomato (Solanum lycopersicum L.). By contrasting a tolerant (A10) and a sensitive (M56) genotype, we demonstrate that superior resilience is conferred by a coordinated activation of MAPK-ethylene signalling pathways, significant accumulation of key osmolytes (proline, GABA, glutamate) and polyphenols, and the stringent maintenance of ion homeostasis through the selective upregulation of critical transporters like SlHKT1;2, which restricts Na⁺ shoot accumulation. The second study provides the first integrated multi-omics characterisation of the phosphate (Pi) starvation response (PSR) in sugar beet (Beta vulgaris L.). Pi deficiency induced significant biomass reduction, altered root architecture, and triggered distinct spectral reflectance signatures. Transcriptomic analysis revealed extensive tissue-specific reprogramming, with key responses including the upregulation of the SPX-PHR signalling module, high-affinity Pi transporters (e.g., PHT1;7), purple acid phosphatases (PAPs), and genes involved in oxidative stress protection, carbohydrate metabolism, and root cell wall remodelling. This work elucidates the highly coordinated PSR strategy in sugar beet and provides valuable genetic resources. The third study provides a rigorous validation of a novel class of lignosulfonate-based humic substance (LB-HS) biostimulants for enhancing Nitrogen Use Efficiency (NUE) in maize (Zea mays L.). We demonstrate that specific LB-HS formulations act as powerful biostimulants by synergistically stimulating root growth, activating the core nitrogen metabolism enzymes (nitrate reductase, glutamine synthetase, glutamate synthase), and upregulating the expression of high-affinity nitrate transporters (ZmNRT2.1, ZmNRT2.2) and their essential accessory protein (ZmNAR2.1), thereby establishing a direct cause-effect model for improved N uptake and assimilation. Collectively, this thesis provides fundamental and applied insights into the mechanisms of plant stress tolerance and biostimulant function. The findings contribute to the development of targeted strategies for breeding more resilient crops and for integrating evidence-based biostimulant formulations into sustainable crop management frameworks, ultimately supporting the advancement of productive and low-environmental-footprint agricultural systems.